Melatonin effects on hard tissues: bone and tooth.
Melatonin is an endogenous hormone rhythmically produced in the pineal gland under the control of the suprachiasmatic nucleus (SCN) and the light/dark cycle. This indole plays an important role in many physiological processes including circadian entrainment, blood pressure regulation, seasonal reproduction, ovarian physiology, immune function, etc. Recently, the investigation and applications of melatonin in the hard tissues bone and tooth have received great attention. Melatonin has been investigated relative to bone remolding, osteoporosis, osseointegration of dental implants and dentine formation. In the present review, we discuss the large body of published evidence and review data of melatonin effects on hard tissues, specifically, bone and tooth.
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Bone remolding processes are mediated by hormones, cytokines, growth factors and other molecules . One of the hormones modulating bone formation and resorption is melatonin. It is hypothesized that melatonin, perhaps through three principle actions, modulates bone metabolism. Firstly, melatonin directly affects the actions of osteoblast and osteoclast. Numerous studies documented that melatonin increases pre-osteoblast/osteoblast/osteoblast-like cell proliferation, promotes the expression of type I collagen and bone marker proteins (e.g., alkaline phosphatase, osteopontin, bone sialoprotein and osteocalcin), and stimulates the formation of a mineralized matrix in these cells [23–27]. Besides, melatonin inhibits the differentiation of osteoclasts via decreases in the expression of RANK mRNA and increases in both the mRNA and protein levels of osteo-protegerin [28,29]. Secondly, melatonin indirectly regulates bone metabolism through the interaction with systemic hormones (e.g., PTH, calcitonin, and estrogen) or other moleculars. Ladizesky et al.  revealed that estradiol treatment could prolong the effect of melatonin to augment bone remodeling in ovariectomized rats; it indicates that appropriate circulating estradiol levels might be needed for melatonin effects on bone. Thirdly, osteoclasts generate high levels of superoxide anions during bone resorption that contribute to the degradative process. Melatonin is a significant free-radical scavenger and antioxidant. It can clear up the free radicals generated by osteoclast during the bone resorption process and protect bone cells from oxidative attacks [18,30,31].
Some studies revealed the possible etiologic role of melatonin in osteoporosis. Nocturnal plasma melatonin levels decline with age. It has also been reported that melatonin secretion decreases sharply during menopause, which is associated with post-menopausal osteoporosis [46,47]. A correlation between decreased plasma melatonin levels and an increased incidence of bone deterioration as seen in post-menopausal women has been examined . Furthermore, Ostrowska et al.  found that a pinealectomy in rats promotes the induction of bone metabolism biomarkers. In addition, Feskanich et al.  reported that twenty or more years of nightshift work significantly increased the risk of wrist and hip fractures in post-menopausal women. Nightshift work leads to disturbances of melatonin secretion as well as severe circadian rhythm disruption. These observations taken together suggest that melatonin may be involved in the pathogenesis of osteoporosis.